1. Field of the Invention
The present invention relates to the connection of pipe. More particularly, it relates to the connection of submerged flowlines and pipelines used for the transportation of oil and gas from offshore wells and production facilities.
2. Description of the Prior Art
The development of offshore oil fields in deep water has necessitated the laying of lengthy pipelines to transport oil and gas from remotely located fields to more centrally located production facilities and storage terminals. A problem frequently encountered in laying pipeline is the making of underwater pipeline connections particularly if the connection is not diver assisted. If a pipeline is laid over great distances and in deep water, it is difficult to precisely position the end of the pipeline at the desired termination point. It is especially difficult to accurately position the ends of pipelines to be connected. In most instances, a correction in length and misalignment must first be made to make the final pipeline connections.
Over the years many attempts have been made to develop simple and reliable methods for connecting underwater pipelines. One conventional approach has been to lay the pipelines along a predetermined path so that they terminate as close as possible to the point of connection. However, because of the unpredictable sea floor contours and the difficulty in maintaining a precise path in laying the pipelines, there are invariably both spatial and axial misalignments between the ends of the pipelines.
One technique frequently used to correct for such misalignment is to insert a spool piece between the ends of the pipelines. Normally, the spool piece is a specially designed section of pipe which is accurately fabricated so as to correct for spatial or axial misalignment. After it is fabricated, the spool piece is interconnected between the ends of the pipelines. In shallow waters, joinder of the spool piece with the pipelines is typically accomplished with the assistance of divers who weld or flange-connect the spool piece to the pipelines' ends. Beyond demonstrated diver capability depths of about 500 feet, the spool piece must either be remotely inserted between the pipelines or the ends of the pipelines must be retrieved for an above-surface connection. However, remote installation techniques for inserting and connecting the spool piece using subsea, unmanned manipulator devices require an accurate alignment of the ends of the pipelines. In addition, an above-surface connection is a time-consuming and expensive operation.
An improvement over the above-surface connection is set forth in U.S. Pat. No. 3,670,513 (Matthews). In this patent, a method is disclosed for connecting an underwater pipeline to another conduit or pipeline at a fixed subsurface location. The method involves determining the subsurface distance and orientation between the free ends of a first and second pipeline which are to be connected. The free end of the first pipeline is then raised to the surface and a lateral extension which spans the subsurface distance between the free ends is attached. The first pipeline and the lateral extension are then lowered back into the water and guided into alignment with the free end of the second pipeline. A connection is then made between the lateral extension and the second pipeline's free end, thus completing the flowline connection. Since the lateral extension is connected to an end of one of the pipelines, a precise fit for the extension is not necessary. A slight misalignment can be adjusted for by moving the pipeline and attached extension along and transverse to the horizontal axis of the other pipeline until the extension is properly aligned.
The method disclosed by Matthews requires an approximate alignment between the pipelines' ends after the lateral extension is attached. Misalignments occasionally require lengthy movement of the pipelines and extension. Such a movement may be difficult to make, particularly in deep water. In addition, Matthews requires that the first pipeline be retrieved to connect the lateral extension. However, retrieving the pipeline may be difficult in deep water. Therefore, there is a need in the art for a further improved system to make subsea pipeline connections.